In conventional gas turbines, acoustic oscillation usually occurs in the chambers of the gas turbines. The term chamber means any gas volume where combustion dynamics exist. In these chambers a gas (for example a mixture of fuel and air or exhaust gas) flows with high velocity which causes noises. Burning air and fuel in the combustion chamber causes further noises. This acoustic oscillation may evolve into highly pronounced resonance. Such oscillation, which is also known as combustion chamber pulsations, can assume amplitudes and associated pressure fluctuations that subject the combustion chamber itself to severe mechanical loads that may decisively reduce the life of the combustion chamber and, in the worst case, may even lead to destruction of the combustion chamber.
To reduce the acoustic oscillations noise it is well known in the art to install acoustic damping devices like a Helmholtz resonator, a half-wave tube, a quarter-wave tube or other types of damping devices.
As an example, FIG. 1 schematically shows the arrangement of a conventional Helmholtz resonator. As shown in the figure, the Helmholtz resonator 10 comprises a resonator cavity 11 connected to the chamber 14 via a neck 12. The neck 12 includes a mouth 13 at its outlet end. The chamber 14 is only partially shown in FIG. 1 with the inner surface 15 of the chamber.
The principle of the Helmholtz resonator is given herein. The resonator cavity 11 acts as a spring for air expansion and contraction in the cavity. The air in the neck 12 behaves as a mass connected to the spring. This system has one or more resonance frequencies. When acoustic waves are at a frequency close to one of the resonance frequencies of the damper and impinge the mouth of the neck, the damper reduces or damps such pulsations. The mass of air oscillates due to the spring effect of the cavity. The oscillation of the air through the neck triggers vortex shedding at the neck. In this way, acoustic energy is converted into aerodynamic energy which ultimately dissipates into heat. Enhanced dissipation is often introduced by means of a flow of gas through the damper neck. This is referred to through flow or bias flow.
However, the gas flow inside the chamber flowing across the mouth 13 of the neck 12 (referred to as grazing flow in the following description) will affect the damping performance. In particular, the inventors of this invention have found that, a high velocity of the grazing flow compared to the air flow through the damper neck (referred to as bias flow in the following description), has a detrimental effect on the damper performance. To avoid the decreased performance of the damper, current solution is to arrange the damper in the regions of the chamber where the velocity of the grazing flow is not so high compared to the bias flow velocity. However, under some situations, the preferred damper location is in regions subjected to high grazing flow velocity.